The present invention pertains generally to heaters and more particularly to radiant heaters.
Generally, three methods exist for heating articles in an industrial process, i.e., convection, conduction, and radiant heating. A major disadvantage of conduction heating is that it requires heater contact with the article, which can result in a slow and non-uniform heating process. Also, overall operating efficiency is low, in many cases, depending upon the efficiency of conduction to the particular material to be heated.
Convection heating, such as used in convection ovens, provides good uniform heating throughout the article and utilizes relatively inexpensive fossil fuels. However, as in conduction heating, the overall operating efficiency is low due to poor coupling of heat to the article. Efficiency is also reduced by the losses inherent in fuel combustion and the necessity to ventilate the convection oven to remove by-products produced during heating. Ventilation of convection ovens requires removal of the gaseous heating medium, which further reduces efficiency. Moreover, convection ovens are normally employed as batch type heaters which require extra handling in both automated and manual processes. Also, convection ovens require a substantial preheating time to obtain a suitable operating temperature, resulting in the necessity to maintain operating temperatures in the oven during short periods of non-use when it would be impractical to turn the oven off because of excessive delays. This, again, reduces overall efficiency.
To overcome these disadvantages and limitations, radiant heaters have been utilized in industry which employ nichrome and tungsten filaments in quartz tubes. Both aluminum and gold reflectors are used in conjunction with these sources to increase efficiency. Although radiant heaters are more expensive to operate than other types of heaters during actual operating time, a number of factors result in actual lower overall operating cost when radiant heaters are implemented in an industrial process. For example, since the energy coupling medium utilized is primarily infrared radiation, ventilation to remove by-products produced during the heating process does not result in removal of the heating medium, such as in convection ovens. Similarly, the energy from radiant heaters can be coupled very efficiently to radiant absorbers. The process of coupling energy to the radiant absorber can be controlled with high accuracy such that only the required energy to heat the article is used so as to further increase efficiency.
Radiant heaters also have the advantage of being adaptable to automated systems due to the small size of radiant heaters which allows easy incorporation into a production line process. Additionally, radiant sources are capable of reaching full operating power within seconds, allowing them to be turned off when production stops or between heating cycles in the automated process. Also, since a radiant source is capable of coupling large amounts of heat into an article in seconds, it can be incorporated in a high speed automated process without limiting production. Moreover, radiant heaters are capable of providing both uniform heating as well as localized heating. The source of power for the radiant source is electricity which is a clean and safe source of power having a high conversion efficiency, i.e., near 90%, to ir radiation.
Typical prior art radiant heaters utilize curved reflectors, such as parabolic reflectors, with the radiant source located at the focal point of the parabola. Although this increases the amount of radiation which impinges on the article, a large portion of the reflected radiation can be blocked by the radiant source, such as illustrated by radiation 11 of FIG. 1. Additionally, in many reflector designs a large portion of radiation emitted by the source does not impinge directly upon the article to be heated and also is not reflected by the parabolic reflector to impinge upon the article to be heated, such as illustrated by radiation 13 of FIG. 1. Consequently, a large portion of the energy emitted by a radiant source in typical radiant heater designs is unusable. Furthermore, it is occasionally useful to refract light through an article to be heated to increase the amount of absorbed energy.
Consequently, it would be desirable to provide a radiant heater which is capable of directing a large portion of the energy emitted by a radiant source onto one or more articles to be heated to increase efficiency of the system.